The present invention is directed toward rubber articles with improved electrical conductivity. The invention has particular application for timing belts to obtain timing belts with improved electrical conductivity properties.
In devices that utilize rubber in conjunction with mechanical drive means, the movement of the belt relative to the other parts of the apparatus builds a static electrical charge on the belt. Other mechanical parts of the apparatus can also generate a static charge that may be transferred to the belt. In some applications, the presence of the static charge may damage sensitive electrical equipment. Static charge build-up also may limit the operating speed of the belt, thus limiting the operation speed of the device. By forming a conductive belt, the static charge is dissipated during movement, preventing a charge buildup that could lead to the generation of sparks or arcs. This also permits an increased operation speed.
A concern in forming conductive belts is maintaining the abrasion properties of the belt and the ability of the conductive material to bond and adhere with the other belt components. To that end, static conductivity in a belt has been obtained by numerous methods.
U.S. Pat. No. 4,823,942 discloses an electrically conductive belt comprising a layer of electrically conductive rubber containing conductive carbon black. Similarly, U.S. Pat. No. 4,758,213 discloses a composite belt with a layer of conductive rubber. The layer has 20 to 40 parts by weight of a conductive material selected from carbon black, metallic powder, metallic fibers, or conductive organic fibers. U.S. Pat. No. 4,767,389 discloses a driving belt reinforced with multi-functional threads, including electrically conductive filaments of metal or carbon fibers, providing the belt with anti-static properties. U.S. Pat. No. 5,417,619 discloses a v-ribbed belt with an outer rubber layer formed of a rubber composition including conductive carbon.
U.S. Pat. No. 4,392,177, GB 2,100,390, and WO 97/47460 disclose forming a roller with an outer thermoplastic layer. The roller of U.S. Pat. No. 4,392,177 and GB 2,100,390 disclose a metal roller with an outer layer of plastic material that contains pulverized metallic material to create a surface reistance. WO 97/47460 discloses a method of applying a layer of heat shrinkable polymer material onto a machinery roller.
However, when the belt is formed with an outer layer of thermoplastic with good insulation properties, such as polyethylene, known methods for providing electrical conductivity in the belt prove insufficient to yield the desired properties. The present invention is directed toward forming an electrically conductive belt wherein the composition of the outer thermoplastic layer is utilized to provide the desired physical probes, including electrical conductivity, adhesion, abrasion and flexibility for both operation and tooth formation.
Additionally, under current standards for xe2x80x9celectrically conductive beltsxe2x80x9d, it is only required that the belt have an initial ohm rating within a range that is determined to be electrically conductive. There is no set requirement that the belt maintain the electrical property after any period of belt life. The present invention is also directed toward a belt that can be made to maintain a desirable electrical conductivity during the life of the belt.
The present invention is a translational article including a material in the article that tends to build an electrical charge during normal use when in contact with a second article with which the article has relative movement. The translation article has a body and a cover layer on the innermost surface. The innermost surface is the article surface which contacts the second article. The cover layer of the inventive article is an electrically conductive thermoplastic.
Another aspect of the present invention is that the cover layer is ultra high molecular weight polyethylene.
Another aspect of the present invention is that the cover layer has a thickness within the range of 0.05 to 0.25 mm (0.002 to 0.01 inches).
In another aspect of the present invention a bonding layer may be provided adjacent the cover layer.
In another aspect of the invention, the article has an initial surface electrical resistance of not more than 0.05 M ohms.
In another aspect of the invention, the article is a belt. The belt may be of any conventional body construction, and may include any one or multiple conventional elements such as teeth, reinforcing tensile members, and reinforcing fabric.